1. Field of the Invention
The present invention relates to an imageable element comprising a substrate and a thermally imageable composition coated on a surface of the substrate. The thermally imageable composition comprises a thermally sensitive polymer, which comprises at least one covalently bonded unit and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each non-covalently bonded unit. More particularly, the present invention relates to a thermally sensitive supramolecular polymer prepared from a hydrocarbyl-substituted isocytosine, a diisocyanate and a polyfunctional material such as phenolic resin, acrylic resin, polyester resin and polyurethane resin.
2. Description of the Prior Art
WO 98/14504 discloses a supramolecular polymer containing monomeric units, which in pairs, form at least 4 H-bridges with one another. The article in J. Org. Chem., 53, 5787-9 (1988) discloses bifunctional compounds that can associate into polymers or oligomers by 2-center H-bond units. Supramolecular polymers based on 3-center H-bond units are disclosed in Macromolecules, 28, 782-83 (1995). None of the above references discloses the use of these polymers in thermal imaging.
EP 969 966 discloses phenolic polymers together with a non-photosensitive solubility inhibitor which provides acceptor sites for H-bonding and EP 985 166 discloses the corresponding methods. The present invention is directed to a supramolecular polymer having non-covalent interactions, such as H-bonding, as an integral part of the polymer structure. The H-bonding in the present invention is not between a H-donor polymer and a solubility inhibitor having H-acceptor sites, but is an integral part of the polymer structure.
WO 99/01795 discloses phenolic polymers, which are modified with groups that provide acceptor sites for H-bonding with other phenolic polymers The present invention is directed to a supramolecular polymer having non-covalent interactions as an integral part of the polymer structure.
An article by B. J. B. Folmer et al., Advanced Materials, Vol. 12 (No. 12), pages 874-878 (2000), discloses supramolecular polymer materials. The article does not disclose polymers derived from polyfunctional materials, such as, polyfunctional phenolic resin, acrylic resin, polyester resin or polyurethane resin, nor does it disclose the use thereof in thermal imaging.
The present invention includes an imageable element comprising a substrate and a thermally imageable composition according to the present invention coated on a surface of the substrate. The thermally imageable composition according to the present invention comprises: a thermally sensitive polymer which exhibits an increased solubility in an aqueous developer solution upon heating, said thermally sensitive polymer comprising: at least one covalently bonded unit; and at least one thermally reversible non-covalently bonded unit, which includes a two or more centered H-bond within each non-covalently bonded unit.
The present invention further includes a method of producing an imaged element. The method comprises the steps of providing an imageable element comprising a substrate and a thermally imageable composition according to the present invention coated on a surface of the substrate; exposing the imageable element to thermal radiation to produce imagewise exposed regions; and contacting the exposed imageable element and a developer within a period of time after the exposing step to remove the exposed regions and thereby produce the imaged element.
The present invention still further includes a process for preparing a supramolecular polymer. The process comprises contacting a hydrocarbyl-substituted isocytosine and a diisocyanate to produce an isocytosine/isocyanate mono-adduct; and contacting the isocytosine/isocyanate mono-adduct and a polyfunctional material selected from the group consisting of: polyfunctional phenolic resin, acrylic resin, polyester resin, polyurethane resin, and a combination thereof, at a temperature and for a period of time sufficient to produce the supramolecular polymer. In addition to the thermally imageable composition, the present invention includes a monomer represented by the formula: 
wherein each R1 and R2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO)2; R3 is a phenolic residue derived from a polyfunctional phenolic resin represented by the formula R3(OH)n; and n is at least 1.
The present invention also includes a supramolecular polymer derived from monomers represented by the formula: 
wherein each R1 and R2 is independently selected from the group consisting of: hydrogen and hydrocarbyl; Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO)2; R3 is a phenolic residue derived from a polyfunctional phenolic resin represented by the formula R3(OH)n; and n is at least 1.
The present invention further includes an isocytosine/isocyanate mono-adduct in a monomeric or quadruple hydrogen bonded dimeric form. The monomeric form of the isocyanate mono-adduct can be represented by the formula: 
wherein each R1 and R2 is independently selected from the group consisting of: hydrogen and hydrocarbyl and Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO)2, the diisocyanate being selected from the group consisting of isophorone diisocyanate, methylene-bis-phenyl diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, tetramethylxylylene diisocyanate, dimers thereof, adducts thereof with diols, and mixtures thereof.
The present invention still further includes an isocytosine/isocyanate bis-adduct in a monomeric or quadruple hydrogen bonded polymeric form. The monomeric form of the bis-adduct can be represented by the formula: 
wherein each R1 and R2 is independently selected from the group consisting of: hydrogen and hydrocarbyl and Y is a hydrocarbylene derived from a diisocyanate represented by the formula Y(NCO)2.
In the present invention, the image areas are reinforced with strong two or more centered H-bond links. Therefore, the image areas exhibit long press life and increased resistance to press chemicals when compared with the systems of the prior art, which include relatively weak single H-bond units. Thus, the thermally reversible polymers of the present invention can have relatively strong bonds and provide thermal solubilization systems that have enhanced differentiation between exposed and non-exposed areas. These polymers can undergo thermal solubilization and provide a durable, non-exposed area, corresponding to the etch resist in a PCB or to the image area in a printing plate.
The imagewise exposure and development steps of the present invention do not require an intermediate pre-development heating step. In addition, superior press life is achieved without the need of a post-development bake because of the strength of the two or more centered H-bond links, such as, 4-centered H-bond units, in the image areas. The superior press life resulting from the strongly bonded image is unexpected for a no-preheat, digitally imaged composition. Furthermore, the inventive element of the present invention, particularly when it comprises a single layer coating composition, is simple to manufacture. Accordingly, the present invention provides thermally reversible polymers that can be used in simple and cost effective methods that are useful in thermal imaging of lithographic plates and circuit boards.